MH's ideal coil and voltage question

[hoptoad]

snip...
What that means is that over time the net EMF applied to the inductor decreases, and the CEMF generated by the inductor in response to the decreasing applied EMF is equal and opposite.
snipp...
I will repeat to you, a voltage drop across any component is actually a CEMF.  However, for sure it makes sense to try and keep the nomenclature simple and understandable for all.
MileHigh

There is never a decrease or increase in the APPLIED EMF (voltage/current) source after connection. Remember - its an ideal voltage/current source to feed the ideal inductor.
Any variables like the varying resulting current through the inductor and its magnetic field strength are due to the variability of the emergent cemf that arises from the ideal Applied EMF. If the cemf was a steady value, all other factors would also be steady.

Of course in the real world unless your supply is very low impedance, when you connect an unloaded voltage source to a load, be it resistor or inductor the applied voltage source itself will fall slightly. How much it falls depends on the capacity of the source to deliver the required current into the load at a given maintained voltage.

Messing with the nomenclature seems to be the source of most of the tension in the discussion. There are times when it fails to communicate the concept or nuance of a particular issue, but that's when its time to look for a unique new descriptor to avoid confusion, not apply a well understood and widely used description of a phenomenon to another in which agreement and usage of another term is already common and well established.

Yes we use ideal equations to design circuits, work out currents, voltages etc.. We also know that our circuits and calculations contain error margins due to the fact that none of our components are ideal. We don't build perfect circuits but we still build good ones that are accurate enough for the tasks they are designed for.
Cheers

[citfta]

I am going to repost this until you reply to it.  You have clearly gotten confused between voltage drop and CEMF.

MileHigh,

You are apparently using the voltage drop across the coil as YOUR definition for CEMF.   That is not what the rest of the electronics world uses as that definition.  For the rest of us CEMF is the generated voltage that opposes the applied voltage.  The CEMF is generated by the increasing magnetic field of the coil as the current rises.  If your claim that the CEMF equals the EMF were true then no current would flow and that means the could NOT BE any CEMF.  Sorry, but your argument makes no sense at all.  I haven't read all the posts in this thread but it appears you are the only one that believes CEMF can equal EMF.  I really don't think all the rest of us are wrong and you are the only one correct about this.

Respectfully,
Carroll

[MileHigh]

I strongly disagree. A voltage drop across a resistor is simply that. It can not be considered a "source" of emf or cemf. A resistor dissipates energy supplied by an emf. See the attached definition by Hyperphysics.

An emf is a source of energy. The voltage measured across a resistor is an indication of the dissipation of energy. Let's not confuse the two.

I am not going to disagree with you strongly but I will add one caveat.  For starters, I already mentioned that several searches used the term "potential difference" for a resistor in a current loop with an EMF source.  So, indeed, a resistor is not a source of direct energy, but there is a tangible voltage associated with it.  "A source or perhaps instance of potential difference (when current is flowing through the device)" is a reasonable thing to say.

Let me explore the caveat.  Let's look at MOS-type semiconductors for a second.  When you get into the details of how they are constructed and how they function, it quickly becomes apparent that it's all about electrons and the moving about of electronics.  It starts to become an impediment to talk about current flow in these devices because it just gets too damn cumbersome.  So that's why they use the terms "source" and "drain" in that realm.  That refers of course to a source of electrons and a drain for electrons.

If you are in the realm of academic electrical engineering research, and looking at things on a deeper level, you may indeed use a different nomenclature.  I am talking purely hypothetically here.  When you start looking at voltage in detail, and you are snaking your way around a loop and observing the electric field, what do you see?  You typically observe very weak electric fields in wires, strong electric fields in capacitors, and as you snake your way through a resistor, depending on the value and the current flow, you can observe say a moderate or a very strong electric field.  In that realm of academic electrical engineering research, it may indeed be very convenient to refer to a resistor as a CEMF source because when you pass through it you can "go downhill" in terms of the electric field.

The realm of electronics and electrical engineering is so wide and so huge that different sectors will use their own rationalized units and use their own preferred nomenclature for devices and variables, etc.

So I am not "pushing" to say a resistor is a CEMF source, I am just saying that it may be valid to say that even if in this realm it's not an appropriate term.

MileHigh

[hoptoad]

I strongly disagree. A voltage drop across a resistor is simply that. It can not be considered a "source" of emf or cemf. A resistor dissipates energy supplied by an emf. See the attached definition by Hyperphysics.

An emf is a source of energy. The voltage measured across a resistor is an indication of the dissipation of energy. Let's not confuse the two.

Thanks for posting that. In referring to nomenclature, that screenshot raises an issue which I had to deal with some years ago in trying to convey the nuance of difference between generating cemf and using it. As the picture says, EMF (CEMF)is not a force it is a potential.

For the purpose of explaining a concept I instead referred to the accepted CEMF as CEMP (Counter electromotive potential) to convey the difference between an unloaded flyback voltage, and CEMF (Counter electromotive force) comprising a loaded CEMP resulting in flyback current. Outside the particular examples I was trying to explain, I would not use the term. But as I said, occasionally the nomenclature we use lacks nuance without added descriptors. For example, BEMF or CEMF is a term that is widely used to describe a voltage that can be induced in a number of different ways that doesn't give clear insight into the specific method of its creation. Is it self induced back emf, rotor magnet induced back emf, collapsing field induced back emf. It is all the same thing but the mechanisms vary, while the name gives little nuance to any specific method of production.

The confusion can really become quite chaotic in pules motor operation descriptions because you have cemf that arises from the applied emf simply due to the inductor characteristics. Then you have cemf induced from the rotor magnets, and also cemf from the collapse of the magnetic field when the emf is removed again. If we had accepted unique names for the cemf according the method of creation, it would simply make it easier to understand and negate the need for additional descriptors to avoid confusion. Underlying all the methods is one commonality - changing current/magnetic fields. But a broader vocabulary would help the description process.

Cheers

[MileHigh]

MileHigh,

You are apparently using the voltage drop across the coil as YOUR definition for CEMF.   That is not what the rest of the electronics world uses as that definition.  For the rest of us CEMF is the generated voltage that opposes the applied voltage.  The CEMF is generated by the increasing magnetic field of the coil as the current rises.  If your claim that the CEMF equals the EMF were true then no current would flow and that means the could NOT BE any CEMF.  Sorry, but your argument makes no sense at all.  I haven't read all the posts in this thread but it appears you are the only one that believes CEMF can equal EMF.  I really don't think all the rest of us are wrong and you are the only one correct about this.

Respectfully,
Carroll

If you just read my long posting about the term "CEMF" applied to resistors then you will see there is a similar discussion that can be had about your posting above.

A voltage drop across a coil is not MY definition of CEMF.  The simple fact is that they are synonymous.  In the realm of electronics often enough there are multiple ways to state the same thing.  There is nothing wrong with that, often a givien identical situation can be viewed from differing perspectives using different terminology that in the long run all mean exactly the same thing.

For the rest of us CEMF is the generated voltage that opposes the applied voltage.

Yes, and from the battery's perspective that generated voltage is causing the battery's voltage to drop through the coil.  So the CEMF and the generated voltage that opposes the applied voltage and the voltage drop across the coil are all exactly the same thing.

If your claim that the CEMF equals the EMF were true then no current would flow and that means the could NOT BE any CEMF.

Yes, I know, we are still in zombie territory.  Just like you can have multiple terms that mean exactly the same thing, you can have a device that behaves in a way where it is like two things happen at the same time.

In a coil, the CEMF is being generated as a result of the changing current flow through the coil (v = L di/dt) and this is all talking place because the battery is imposing EMF across the coil.  Everything is awesome and it's all happening at the same time and it's all perfectly normal.  It's NORMAL and nobody should be even flinching or questioning it.  I suspect the root cause of the hysteria is "Brad's disease."  Brad says something completely wrong and a bunch of zombie sycophants follow along because for some strange reason you latch onto what he says.  THINK FOR YOURSELF, and wake up.  You connect a battery to a coil and you measure 12 volts at the junction of the battery and the coil.  By definition the EMF is 12 volts and by definition there is changing current in the coil generating a counter EMF of 12 volts.  This is not rocket science, this is basic electronics.

The EMF and the CEMF are the same damn thing!  The battery says, "I am imposing 12 volts across you."  The coil says, "Oh shit, then I have to let changing current flow through me at a rate where I muster up the same 12 volts."   They are the SAME THING.  They both measure 12 volts with a volt meter and have the same polarity if you use the same ground reference.  They HAVE to be the same potential because they are CONNECTED to each other.

If they are the same damn thing then why is one called CEMF?  It's because you "travel though the loop" in ONE DIRECTION only.  So if you go clockwise and you go UP in potential because of the EMF, then as you continue on your journey through the coil you go DOWN in potential.  Hence the "counter."  You go up in potential and then you counter that by going down in potential.  But when you are not "in the loop" the EMF and the CEMF are EXACTLY THE SAME with the SAME polarity.

I will say again, there is nothing really to debate here.  We are all zombies trapped in a Monty Python sketch.  We need to bust out.

MileHigh